Hydrangea + Ecology

Mt Cuba just published the results of their native Hydrangea Trials and found Hydrangea arborescence ‘Haas Halo’ to be the cultivar with the best ornamental performance, while also providing nectar and pollen for visiting insects. That latter point is an important flag, as many, if not most, Hydrangea cultivars offer little to pollinators.

Mt Cuba found that lacecap hydrangeas had higher insect visitors than mopheads because lacecaps have an abundance of fertile flowers in the center, providing pollen and nectar to visitors. Mophead flowerheads - like ‘Annabelle’ and ‘Incrediball’ - have far fewer fertile flowers, opting instead for sterile, showy sepals that attract a lot of attention (much of it from us) but don’t provide food.

On mopheads, hungry insects have to work extra hard to find flowers with food, climbing around laboriously while exposing themselves to the dangers of foraging. Look how hard this bumblebee is working! Mopheads are stunning, and I wouldn’t want to garden without them. But I find that adding a few lacecaps alongside them is a solid strategy.

Of course hydrangeas aren’t just a buffet for hungry bees. They also provide homes to stem-nesting bees and food for the caterpillars who eat their leaves. When considering how to support the bees and moths that use hydrangea as their nesting and host plant, I found that it made for a solid illustration of how to do the research part of ecological horticulture. The below covers the resources I used and the steps I took with the hope that you might use them in your practice.

Many of us are now aware of the importance of stems with pithy or hollow centers as homes for stem-nesting bees. Hydrangea stems are prefect and well-loved, with a hole large enough for bees to overwinter or make nests within. So when cutting stems back, if you have to, be sure to leave them at least 18” for the bees to use. Heather Holm calls this “stem stubble,” which is adorable.

But in addition to stems and bees, we must also consider who might be eating the leaves. Here is a website that allows you to write in a plant name, really any plant in the world it seems, and it will tell you which butterflies and moths use that plant as a host. When I put in Hydrangea arborescence, I find that Darapsa versicolor, the Hydrangea Sphinx Month, uses the shrub as a host plant. And what a stunning beauty.

Photo from iNaturalist by nolieschneider (CC BY-NC)

I then go to www.ButterfliesAndMoths.org to find out more information about the species. I can read about lifecycle, range, and AHA! “Fully-grown caterpillars pupate under leaf litter in loose cocoons of leaves and silk.” This is what we’re after. This tells me that it’s especially important to leave leaves under Hydrangea arborescence.

Googling around can also be helpful. I found this awesome site on Sphinx Moths as well as another moth that might be considered a “pest,” as they deform the hydrangea leaves. But let’s look at this. Olethreutes ferriferana, the Hydrangea Leaftier, is a cute little guy that uses living hydrangea leaves to make their homes.

They use silk to tie young leaves together in a “leaf purse” where they can feed and grow safely. Their range overlaps Hydrangea arborescence’s range, meaning these moths likely evolved alongside these plants, specializing on them. It’s as magical a relationship as monarchs and milkweed. Why would we consider these moths “pests” when they just need two leaves of our shrubs? Oh? Did you need those two specific leaves? I really love Doug Tallamy’s “Ten Foot Solution to Pest Problems,” wherein you take ten steps backwards and see if it’s still a problem.

So now we know who eats the leaves of our smooth hydrangeas. And we want to support them because we’re awesome stewards of the land, how do we find out if they live in our region? We go to iNaturalist, arguably one of the most important modern inventions, and look for their distribution. Here’s Hydrangea Sphinx and here’s the Lieftier. We then zoom in to see if they’ve been seen in our area.

And huh, I’m in Brooklyn and the moths are in my region, BUT they do not appear to live in cities. That makes sense, as moths are confused and harmed by the artificial lights that flood cities. So while this would be an amazing find in Brooklyn, chances are that we will not attract this moth. Back to the drawing board. But all of that took <10 minutes. And you can do that for every single plant in your garden! And all the insects you see in your area. Enjoy!